Metal silicides are conductive metal compounds commonly used in the fabrication of integrated circuitry. Exemplary uses are as conductive interfaces to silicon-containing node locations, and as conductive contacts to and conductive strapping layers for field effect transistor gates.
One exemplary low resistance metal silicide is CoSi2. Cobalt silicide also occurs in the monosilicide form (CoSi), but it is the disilicide form which is of greater conductivity and that which is desired to be used in the fabrication of a conductive integrated circuit component. Cobalt silicide can be expressed as CoSix, where “x” typically ranges from 0.3 to 2.
One manner of forming cobalt silicide includes deposition of a layer of cobalt over a silicon-containing material, followed by subsequent high temperature anneal causing interdiffusion of the silicon and cobalt, thereby forming cobalt silicide. Typically and preferably, the cobalt layer is deposited directly on (with “on” in the context of this document meaning in at least some direct physical contact therewith) the silicon-containing material to facilitate diffusion of the cobalt and/or silicon to form the silicide. Less preferred, a very thin native oxide might be received intermediate the cobalt and silicon which disperses during the cobalt silicide formation typically still resulting in adequate cobalt silicide formation. Regardless, the typical annealing is conducted by rapid thermal processing (RTP).
The temperature at which the anneal occurs impacts the degree of formation of one or both of CoSi and/or CoSi2. For example, an annealing temperature of from 500° C. to 550° C. forms substantially all CoSi, and a temperature in excess of 800° C. forms substantially all CoSi2. Intervening temperatures tend to form a mixture of CoSi and CoSi2 including other quantities of silicon with respect to cobalt. Further, such different temperatures are largely determinative of which species, cobalt or silicon, is the predominately moving species. For example at the lower temperatures, cobalt diffusion/movement predominates such that the CoSi which forms tends to form mostly in the silicon region of the substrate, for example elevationally lower in the substrate where a cobalt layer is formed elevationally over silicon. On the other hand at the higher temperatures, silicon diffusion/movement predominates such that the CoSi2 which is formed tends to form mostly in the region of the cobalt layer, for example elevationally higher in the substrate where a cobalt layer is deposited over silicon. Further, where higher temperature anneals are conducted to predominately form CoSi2, the silicon migration can tend to form voids within the underlying silicon-containing substrate beneath where the CoSi2 is formed.
In many instances, it would be desirable to form the CoSi2 in the region prior to the annealing which is predominately composed of silicon movement and also in a manner which prevents void formation. One prior art manner of achieving this is to initially anneal at a lower temperature which forms CoSi lower within the substrate where desired, while also typically leaving some of the cobalt layer unreacted. The cobalt is then stripped by a wet etch, and the substrate subsequently subjected to a high temperature anneal which converts the CoSi to CoSi2. This of course requires two separate annealing steps and stripping of unreacted cobalt prior to conducting the second annealing step. Further, it is highly desirable that unreacted cobalt be stripped from the substrate prior to any subsequent exposure of the substrate to temperatures higher than 650° C. This is because cobalt tends to react with underlying oxide at temperatures greater than 650° C. and the resultant cobalt oxide which is formed can be difficult to remove.
While the invention was motivated in addressing the above identified issues, it is in no way so limited. The invention is only limited by the accompanying claims as literally worded, without interpretative or other limiting reference to the specification, and in accordance with the doctrine of equivalents.